The invention relates generally to transistor amplifier circuits and more particularly, to limiter amplifiers.
Limiter amplifiers provide an amplitude limited output signal in response to a variable amplitude input signal. The input signal is limited to a selected level by the limiter amplifier either by amplifying and limiting or by merely limiting. One purpose of the limiter amplifier is to remove amplitude variations from an input signal so that other signal characteristics such as frequency shift may be processed.
In certain situations, the amplitude of the input signals may vary from microvolts to volts during short time intervals. In many applications which require that these signals be amplitude limited, it is also desirable to have as little variation in phase shift as possible, i.e. where the phase shift through the limiter amplifier does not vary with input signal level. It is desirable that the phase shift through the limiter amplifier does not vary even when there are large changes in the input signal amplitude. This phase shift stability becomes very important in adaptive systems where complex weighting functions are used with quadrature channels.
In applications where both gain and limiting are required of the limiter amplifier at frequencies at or above 100 MHz, prior techniques using bipolar transistor limiter amplifiers have suffered from reduced gain at these frequencies. A reason given for the reduced gain is emitter degeneration caused by the reactance of the inductive component of the transistor emitter source impedance. One common technique used to obtain the gain required at these frequencies is to add more gain stages to the limiter amplifier to compensate for the fall-off of the gain of each stage. An increase in gain stages, however, results in an increase in components, power required, expense, complexity, size and weight.
In addition, where bandpass filtering is also required of the limiter amplifier, prior techniques have employed interstage bandpass filters which typically include discrete inductors. These bandpass filters typically have also had a power loss associated with their use. More gain may be required to compensate for this power loss with the result of more gain stages required and the disadvantages discussed above. Also, where the frequency of operation is relatively low, these discrete inductors are relatively large. The use of these inductors makes implementation of the limiter amplifier in a monolithic integrated circuit more difficult.
Accordingly, it is an object of the invention to provide an improved limiter amplifier.
It is also an object of the invention to provide a limiter amplifier having a bandpass tuning feature which is part of the limiting/amplifying circuitry.
It is also an object of the invention to provide a limiter amplifier having a high degree of phase stability throughout the dynamic range of the input signal.
It is also an object of the invention to provide a limiter amplifier which has higher gain per stage and which is cascadable with other limiter amplifiers.
It is also an object of the invention to provide a limiter amplifier which is better adapted to construction in a monolithic integrated circuit.